1st Edition

The Physics of Solar Energy Conversion

By Juan Bisquert Copyright 2020
    490 Pages 213 Color & 269 B/W Illustrations
    by CRC Press

    490 Pages 213 Color & 269 B/W Illustrations
    by CRC Press

    490 Pages 213 Color & 269 B/W Illustrations
    by CRC Press

    Research on advanced energy conversion devices such as solar cells has intensified in the last two decades. A broad landscape of candidate materials and devices were discovered and systematically studied for effective solar energy conversion and utilization. New concepts have emerged forming a rather powerful picture embracing the mechanisms and limitation to efficiencies of different types of devices. The Physics of Solar Energy Conversion introduces the main physico-chemical principles that govern the operation of energy devices for energy conversion and storage, with a detailed view of the principles of solar energy conversion using advanced materials.

    Key Features include:

    • Highlights recent rapid advances with the discovery of perovskite solar cells and their development.
    • Analyzes the properties of organic solar cells, lithium ion batteries, light emitting diodes and the semiconductor materials for hydrogen production by water splitting.
    • Embraces concepts from nanostructured and highly disordered materials to lead halide perovskite solar cells
    • Takes a broad perspective and comprehensively addresses the fundamentals so that the reader can apply these and assess future developments and technologies in the field.
    • Introduces basic techniques and methods for understanding the materials and interfaces that compose operative energy devices such as solar cells and solar fuel converters.

    Contents

    Preface........................................................................................................................................................................xv

    Acknowledgments....................................................................................................................................................xvii

    Author.......................................................................................................................................................................xix

    Chapter 1 Introduction to Energy Devices...............................................................................................................1

    References...............................................................................................................................................9

    PART I Equilibrium Concepts and Kinetics

    Chapter 2 Electrostatic and Thermodynamic Potentials of Electrons in Materials...............................................13

    2.1 Electrostatic Potential..................................................................................................................13

    2.2 Energies of Free Electrons and Holes.......................................................................................... 14

    2.3 Potential Energy of the Electrons in the Semiconductor............................................................. 17

    2.4 The Vacuum Level....................................................................................................................... 17

    2.5 The Fermi Level and the Work Function....................................................................................20

    2.6 The Chemical Potential of Electrons........................................................................................... 21

    2.7 Potential Step of a Dipole Layer or a Double Layer....................................................................23

    2.8 Origin of Surface Dipoles............................................................................................................24

    2.9 The Volta Potential......................................................................................................................25

    2.10 Equalization of Fermi Levels of Two Electronic Conductors in Contact....................................27

    2.11 Equilibration of Metal Junctions and the Contact Potential Difference......................................28

    2.12 Equilibrium across the Semiconductor Junction.........................................................................29

    General References............................................................................................................................... 31

    References............................................................................................................................................. 31

    Chapter 3 Voltage, Capacitors, and Batteries.........................................................................................................33

    3.1 The Voltage in the Device...........................................................................................................33

    3.2 Anode and Cathode.....................................................................................................................34

    3.3 Applied Voltage and Potential Difference...................................................................................35

    3.4 The Capacitor..............................................................................................................................37

    3.5 Measurement of the Capacitance.................................................................................................38

    3.6 Energy Storage in the Capacitor..................................................................................................40

    3.7 Electrochemical Systems: Structure of the Metal/Solution Interface..........................................40

    3.8 Electrode Potential and Reference Electrodes.............................................................................42

    3.9 Redox Potential in Electrochemical Cells...................................................................................44

    3.10 Electrochemical and Physical Scales of Electron Energy in Material Systems..........................45

    3.11 Changes of Electrolyte Levels with pH.......................................................................................46

    3.12 Principles of Electrochemical Batteries.......................................................................................47

    3.13 Capacity and Energy Content......................................................................................................50

    3.14 Practical Electrochemical Batteries............................................................................................. 51

    3.14.1 Zinc-Silver Battery.......................................................................................................... 51

    3.14.2 Sodium-Sulfur Battery....................................................................................................52

    3.15 Li-Ion Battery.............................................................................................................................. 53

    General References...............................................................................................................................57

    References.............................................................................................................................................57

    Chapter 4 Work Functions and Injection Barriers.................................................................................................59

    4.1 Injection to Vacuum in Thermionic Emission.............................................................................59

    4.2 Richardson–Dushman Equation..................................................................................................60

    4.3 Kelvin Probe Method.................................................................................................................. 61

    4.4 Photoelectron Emission Spectroscopy.........................................................................................63

    4.5 Injection Barriers.........................................................................................................................66

    4.6 Pinning of the Fermi Level and Charge-Neutrality Level...........................................................69

    General References...............................................................................................................................73

    References.............................................................................................................................................73

    Chapter 5 Thermal Distribution of Electrons, Holes, and Ions in Solids............................................................... 75

    5.1 Equilibration of the Electrochemical Potential of Electrons....................................................... 75

    5.2 Configurational Entropy of Weakly Interacting Particles...........................................................76

    5.3 Equilibrium Occupancy of Conduction Band and Valence Band States.....................................76

    5.4 Equilibrium Fermi Level and the Carrier Number in Semiconductors.......................................79

    5.5 Transparent Conducting Oxides.................................................................................................. 81

    5.6 Hot Electrons...............................................................................................................................82

    5.7 Screening.....................................................................................................................................84

    5.8 The Rectifier at Forward and Reverse Voltage............................................................................85

    5.9 Semiconductor Devices as Thermal Machines that Realize Useful Work..................................88

    5.10 Cell Potential in the Lithium Ion Battery....................................................................................90

    5.11 Insertion of Ions: The Lattice Gas Model....................................................................................94

    General References...............................................................................................................................98

    References.............................................................................................................................................98

    Chapter 6 Interfacial Kinetics and Hopping Transitions...................................................................................... 101

    6.1 Principle of Detailed Balance.................................................................................................... 101

    6.2 Form of the Transition Rates.....................................................................................................104

    6.3 Kinetics of Localized States: Shockley-Read-Hall Recombination Model...............................106

    6.4 Reorganization Effects in Charge Transfer: The Marcus Model............................................... 107

    6.5 Polaron Hopping........................................................................................................................ 112

    6.6 Rate of Electrode Reaction: Butler-Volmer Equation................................................................ 115

    6.6.1 Availability of Electronic Species................................................................................. 116

    6.6.2 Availability of Redox Species........................................................................................ 116

    6.6.3 The Kinetic Constant for Charge Transfer.................................................................... 117

    6.7 Electron Transfer at Metal-Semiconductor Contact..................................................................120

    6.8 Electron Transfer at the Semiconductor/Electrolyte Interface.................................................. 121

    General References.............................................................................................................................126

    References...........................................................................................................................................127

    Chapter 7 The Chemical Capacitance.................................................................................................................. 131

    7.1 Carrier Accumulation and Energy Storage in the Chemical Capacitance................................. 131

    7.2 Localized Electronic States in Disordered Materials and Surface States................................. 133

    7.3 Chemical Capacitance of a Single State.................................................................................... 135

    7.4 Chemical Capacitance of a Broad DOS.................................................................................... 136

    7.5 Filling a DOS with Carriers: The Voltage and the Conductivity.............................................. 138

    7.6 Chemical Capacitance of Li Intercalation Materials................................................................. 139

    7.7 Chemical Capacitance of Graphene.......................................................................................... 140

    General References............................................................................................................................. 142

    References........................................................................................................................................... 143

    Chapter 8 The Density of States in Disordered Inorganic and Organic Conductors........................................... 145

    8.1 Capacitive and Reactive Current in Cyclic Voltammetry.......................................................... 145

    8.2 Kinetic Effects in CV Response................................................................................................ 149

    8.3 The Exponential DOS in Amorphous Semiconductors.............................................................150

    8.4 The Exponential DOS in Nanocrystalline Metal Oxides.......................................................... 152

    8.5 Basic Properties of Organic Layers........................................................................................... 156

    8.6 The Gaussian DOS.................................................................................................................... 160

    General References............................................................................................................................. 162

    References........................................................................................................................................... 163

    Chapter 9 Planar and Nanostructured Semiconductor Junctions......................................................................... 167

    9.1 Structure of the Schottky Barrier at a Metal/Semiconductor Contacts..................................... 167

    9.2 Changes of the Schottky Barrier by the Applied Voltage.......................................................... 168

    9.3 Properties of the Planar Depletion Layer.................................................................................. 170

    9.4 Mott–Schottky Plots.................................................................................................................. 171

    9.5 Capacitance Response of Defect Levels and Surface States..................................................... 172

    9.6 Semiconductor Electrodes and the Flatband Potential.............................................................. 173

    9.7 Changes of Redox Level and Band Unpinning.......................................................................... 176

    9.8 Inversion and Accumulation Layer............................................................................................ 180

    9.9 Heterojunctions.......................................................................................................................... 181

    9.10 Effect of Voltage on Highly Doped Nanocrystalline Semiconductors...................................... 183

    9.11 Homogeneous Carrier Accumulation in Low-Doped Nanocrystalline Semiconductors........... 188

    General References............................................................................................................................. 192

    References........................................................................................................................................... 192

    PART II Foundations of Carrier Transport

    Chapter 10 Carrier Injection and Drift Transport.................................................................................................. 197

    10.1 Transport by Drift in the Electrical Field.................................................................................. 197

    10.2 Injection at Contacts.................................................................................................................. 198

    10.3 The Metal-Insulator-Metal Model.............................................................................................202

    10.4 The Time-of-Flight Method......................................................................................................205

    General References.............................................................................................................................206

    References...........................................................................................................................................206

    Chapter 11 Diffusion Transport.............................................................................................................................209

    11.1 Diffusion in the Random Walk Model......................................................................................209

    11.2 Macroscopic Diffusion Equation............................................................................................... 211

    11.3 The Diffusion Length................................................................................................................ 212

    11.4 Chemical Diffusion Coefficient and the Thermodynamic Factor............................................. 213

    General References............................................................................................................................. 215

    References........................................................................................................................................... 215

    Chapter 12 Drift-Diffusion Transport.................................................................................................................... 217

    12.1 General Transport Equation in Terms of Electrochemical Potential......................................... 217

    12.2 The Transport Resistance.......................................................................................................... 217

    12.3 The Einstein Relation................................................................................................................ 219

    12.4 Drift-Diffusion Equations..........................................................................................................220

    12.5 Ambipolar Diffusion Transport................................................................................................221

    12.6 Relaxation of Injected Charge..................................................................................................222

    12.7 Transient Current in Insulator Layers.......................................................................................223

    12.8 Modeling Transport Problems..................................................................................................224

    General References.............................................................................................................................227

    References...........................................................................................................................................227

    Chapter 13 Transport in Disordered Media...........................................................................................................229

    13.1 Multiple Trapping and Hopping Transport...............................................................................229

    13.2 Transport by Hopping in a Single Level...................................................................................231

    13.3 Trapping Factors in the Kinetic Constants...............................................................................233

    13.4 Two-Level (Single-Trap) Model................................................................................................235

    13.5 Multiple Trapping in Exponential DOS....................................................................................237

    13.6 Activated Transport in a Gaussian DOS...................................................................................237

    13.7 Multiple Trapping in the Time Domain....................................................................................239

    13.8 Hopping Conductivity...............................................................................................................241

    13.9 The Transport Energy...............................................................................................................242

    13.10 Variable Range Hopping...........................................................................................................243

    General References.............................................................................................................................245

    References...........................................................................................................................................245

    Chapter 14 Thin Film Transistors..........................................................................................................................249

    14.1 Organic Thin Film Transistors.................................................................................................249

    14.2 Carrier Density in the Channel.................................................................................................250

    14.3 Determination of the DOS in Thin Film Transistor Configuration..........................................252

    14.4 Current-Voltage Characteristics................................................................................................255

    14.5 The Mobility in Disordered Semiconductors............................................................................257

    14.6 Electrochemical Transistor.......................................................................................................258

    General References.............................................................................................................................259

    References...........................................................................................................................................259

    Chapter 15 Space-Charge-Limited Transport........................................................................................................263

    15.1 Space-Charge-Limited Current................................................................................................263

    15.2 Injected Carrier Capacitance in SCLC.....................................................................................265

    15.3 Space Charge in Double Injection............................................................................................267

    General References.............................................................................................................................269

    References...........................................................................................................................................269

    Chapter 16 Impedance and Capacitance Spectroscopies....................................................................................... 271

    16.1 Frequency Domain Measurements...........................................................................................271

    16.2 Dielectric Relaxation Functions................................................................................................272

    16.3 Resistance and Capacitance in Equivalent Circuit Models.......................................................274

    16.4 Relaxation in Time Domain......................................................................................................279

    16.5 Universal Properties of the Frequency-Dependent Conductivity..............................................281

    16.6 Electrode Polarization...............................................................................................................283

    General References.............................................................................................................................284

    References...........................................................................................................................................284

    PART III Radiation, Light, and Semiconductors

    Chapter 17 Blackbody Radiation and Light...........................................................................................................289

    17.1 Photons and Light......................................................................................................................289

    17.2 Spread and Direction of Radiation............................................................................................289

    17.3 Color and Photometry................................................................................................................ 291

    17.4 Blackbody Radiation.................................................................................................................293

    17.5 The Planck Spectrum................................................................................................................294

    17.6 The Energy Density of The Distribution of Photons in Blackbody Radiation..........................295

    17.7 The Photon and Energy Fluxes in Blackbody Radiation...........................................................297

    17.8 The Solar Spectrum...................................................................................................................299

    General References.............................................................................................................................302

    References...........................................................................................................................................302

    Chapter 18 Light Absorption, Carrier Recombination, and Luminescence...........................................................305

    18.1 Absorption of Incident Radiation..............................................................................................305

    18.2 Luminescence and Energy Transfer..........................................................................................307

    18.3 The Quantum Efficiency........................................................................................................... 310

    18.4 The Recombination of Carriers in Semiconductors.................................................................. 311

    18.5 Recombination Lifetime............................................................................................................ 314

    General References............................................................................................................................. 316

    References........................................................................................................................................... 316

    Chapter 19 Optical Transitions in Organic and Inorganic Semiconductors.......................................................... 319

    19.1 Light Absorption in Inorganic Solids........................................................................................ 319

    19.2 Free Carrier Phenomena............................................................................................................323

    19.3 Excitons.....................................................................................................................................325

    19.4 Quantum Dots...........................................................................................................................328

    19.5 Organic Molecules and Materials..............................................................................................330

    19.6 The CT Band in Organic Blends and Heterojunctions.............................................................. 333

    General References............................................................................................................................. 336

    References........................................................................................................................................... 336

    PART IV Photovoltaic Principles and Solar Energy Conversion

    Chapter 20 Fundamental Model of a Solar Cell....................................................................................................343

    20.1 Majority Carrier Injection Mechanisms....................................................................................343

    20.2 Majority Carrier Devices...........................................................................................................344

    20.3 Minority Carrier Devices..........................................................................................................345

    20.4 Fundamental Properties of a Solar Cell.....................................................................................346

    20.5 Physical Properties of Selective Contacts in Solar Cells...........................................................348

    General References............................................................................................................................. 351

    References........................................................................................................................................... 351

    Chapter 21 Recombination Current in the Semiconductor Diode......................................................................... 353

    21.1 Dark Equilibrium of Absorption and Emission of Radiation.................................................... 353

    21.2 Recombination Current............................................................................................................. 355

    21.3 Dark Characteristics of Diode Equation.................................................................................... 356

    21.4 Light-Emitting Diodes............................................................................................................... 357

    21.5 Dye Sensitization and Molecular Diodes...................................................................................360

    General References.............................................................................................................................363

    References...........................................................................................................................................363

    Chapter 22 Radiative Equilibrium in a Semiconductor.........................................................................................365

    22.1 Utilization of Solar Photons......................................................................................................365

    22.2 Fundamental Radiative Carrier Lifetime..................................................................................368

    22.3 Radiative Emission of a Semiconductor Layer..........................................................................369

    22.4 Photons at Nonzero Chemical Potential.................................................................................... 370

    General References............................................................................................................................. 373

    References........................................................................................................................................... 373

    Chapter 23 Reciprocity Relations in Solar Cells and Fundamental Limits to the Photovoltage ........................... 375

    23.1 The Reciprocity between LED and Photovoltaic Performance Parameters.............................. 375

    23.2 Factors Determining the Photovoltage...................................................................................... 378

    23.3 External Radiative Efficiency....................................................................................................382

    23.4 Photon Recycling.......................................................................................................................383

    23.5 Radiative Cooling in EL and Photoluminescence.....................................................................386

    23.6 Reciprocity of Absorption and Emission in a CT Band............................................................387

    General References............................................................................................................................. 391

    References...........................................................................................................................................392

    Chapter 24 Charge Separation and Material Limits to the Photovoltage...............................................................395

    24.1 Light Absorption........................................................................................................................395

    24.2 Charge Separation.....................................................................................................................395

    24.3 Materials Limits to the Photovoltage.........................................................................................398

    General References.............................................................................................................................403

    References...........................................................................................................................................404

    Chapter 25 Operation of Solar Cells and Fundamental Limits to Their Performance..........................................407

    25.1 Current-Voltage Characteristics.................................................................................................407

    25.2 Power Conversion Efficiency.....................................................................................................408

    25.3 Analysis of FF........................................................................................................................... 410

    25.4 Shockley–Queisser Efficiency Limits........................................................................................ 412

    25.5 Practical Solar Cells Efficiency Limits...................................................................................... 413

    General References............................................................................................................................. 419

    References........................................................................................................................................... 419

    Chapter 26 Charge Collection in Solar Cells......................................................................................................... 421

    26.1 Introduction to Charge Collection Properties............................................................................ 421

    26.2 Charge Collection Distance.......................................................................................................422

    26.3 General Modeling Equations.....................................................................................................424

    26.4 The Boundary Conditions.........................................................................................................425

    26.4.1 Charge Extraction Boundary Condition........................................................................426

    26.4.2 Blocking Boundary Condition.......................................................................................427

    26.4.3 Generalized Boundary Conditions................................................................................428

    26.5 A Photovoltaic Model with Diffusion and Recombination........................................................429

    26.6 The Gärtner Model.................................................................................................................... 433

    26.7 Diffusion-Recombination and Collection in the Space-Charge Region.................................... 435

    26.8 Solar Cell Simulation................................................................................................................. 436

    26.9 Classification of Solar Cells....................................................................................................... 437

    26.10 Measuring and Reporting Solar Cell Efficiencies..................................................................... 439

    General References.............................................................................................................................442

    References...........................................................................................................................................442

    Chapter 27 Spectral Harvesting and Photoelectrochemical Conversion................................................................445

    27.1 Conversion of Photon Frequencies for Solar Energy Harvesting..............................................445

    27.2 Tandem Solar Cells....................................................................................................................448

    27.3 Solar Fuel Generation................................................................................................................450

    General References.............................................................................................................................456

    References...........................................................................................................................................456

    Appendix................................................................................................................................................................. 459

    Index........................................................................................................................................................................463

    Biography

    Juan Bisquert is a professor of

    applied physics at the Universitat

    Jaume I de Castello and the funding

    director of the Institute of Advanced

    Materials at UJI. He earned an

    MSc in physics in 1985 and a PhD

    from the Universitat de Valencia

    in 1992. The research work is in

    perovskite solar cells, semiconductor

    optoelectronics, mixed ionicelectronic

    conductors, and solar fuel converters based on

    visible light and semiconductors for water splitting and

    CO2 reduction. His most well-known work is about the

    mechanisms governing the operation of nanostructured

    and solution-processed thin film solar cells. He has

    developed insights in the electronic processes in hybrid

    organic–inorganic solar cells, combining the novel

    theory of semiconductor nanostructures, photoelectrochemistry,

    and systematic experimental demonstration.

    His contributions produced a broad range of concepts

    and characterization methods to analyze the operation

    of photovoltaic and optoelectronic devices. He is a senior

    editor of the Journal of Physical Chemistry Letters. He

    has been distinguished several times in the list of ISI

    Highly Cited Researchers. Bisquert created nanoGe

    Conferences and is the president of the Fundacio Scito.

    He wrote a novel of speculative fiction, The Canamel

    Conjecture.